Ind. Eng. Chem. Res. 2010, 49, 10159–10182
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Fuel Cell Engineering: Toward the Design of Efficient Electrochemical Power Plants Kai Sundmacher* Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany, and Otto-Von-Guericke UniVersity Magdeburg, UniVersita¨tsplatz 2, 39106 Magdeburg, Germany
Fuel cells are electrochemical membrane reactors that are able to convert chemically stored energy directly to electrical energy at high thermodynamic efficiencies. The present paper summarizes the current status and the future needs in fuel cell science and engineering. In the first part, possible primary fuels, alternative fuel processing pathways, and conceptual design aspects of fuel cell systems are discussed. In the second part, important trends in the development of functional materials for the preparation of stable high-performance fuel cells with extended longevity are presented. Thereby, different types of fuel cells are discussed, namely, enzymatic fuel cells (EFCs), alkaline fuel cells (AFCs), polymer electrolyte fuel cells (PEFCs), direct methanol fuel cells (DMFCs), direct ethanol fuel cells (DEFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs). 1. Introduction A long time ago, it was a dream of natural scientists to convert the binding energy that is stored in chemical substances without any detour into electrical energy. In the early 19th century, the experimental investigations and findings of Alessandro Volta (the discovery of the battery principle, 1799) and Michael Faraday (the formulation of electrochemical conversion principles, later named Faraday’s laws, 1832) laid the scientific foundation that enabled Christian Friedrich Scho¨nbein (in 1838) and William Robert Grove (in 1839) to discover the working principles of fuel cells. In 1842, Grove presented a fuel cell battery for continuous conversion of hydrogen and oxygen at two electrodes, accompanied by continuous electrical current. In 1852, he summarized the fuel cell principle in the following famous words: “EVery chemical synthetic action may by a proper disposition of the constituents be made to produce a Voltaic current.” In 1870, these important scientific findings inspired the French poet Jules Verne to write “Water is the coal of the future. The energy of tomorrow is water being decomposed by electrical current. The decomposed elements of water, hydrogen and oxygen, will saVe the energy supply on Earth for an incalculable time period.” Several decades later, namely in 1896, the first worldwide fuel cell patent was obtained by the German chemical technologist Wilhelm Borchers, which was entitled “Process of Transforming Chemical Energy of Fuel into Electrical Energy” (depicted in Figure 1). Instead of the dream fuel of Scho¨nbein, Grove, and Vernesnamely, hydrogensBorchers considered carbon monoxide (CO)-rich gas mixtures, which, at that time, were available in vast amounts. Because of the specific functional principles of the fuel cell, it is possible to convert a large proportion of chemical energy to electrical energy. In the theoretical case of a fully reversible transport of all charged and noncharged species involved, at an operating temperature of 25 °C, the proportion of electrical * Tel.: +49 391 6110 350. Fax: +49 391 6110 353. E-mail address:
[email protected].
Figure 1. Schematic of the first fuel cell patented by Wilhelm Borchers (U.S. Patent 567,959; September 22, 1896).1 In that patent, Borchers states: “... My invention refers to galvanic batteries with two or more cells in which gases obtained by imperfect combustion or by destructive distillation ... of coal and other fuel are oxidized by the oxygen of the air, so as to generate a large amount of the chemical energy of such fuel as electrical energy, using solutions of metals as electrolytes. The gases I have under consideration are carbon monoxide (CO) and all gases containing carbon monoxide, as, for instance, the common coal generator-gas, water-gas, and the escape-gases from blast-furnaces .... The drawing shows a U-shaped vessel V on non-conducting material to hold the electrolyte E. Each compartment holds at the upper end a terminal or electrode, t+ t-, respectively, to connect the electrolyte in the vessel with an external circuit for the electric current to be generated. These terminals may consist of any conducting substance which is not attacked by the electrolyte or by the gases mentioned .... The compartment containing the terminal t+ is provided with a cap c, that contains gas-inlets i and gas-outlets o. Into the cap c, by means of the inlet i, I conduct carbon monoxide or any of the other fuelgases .... The rest of the gas will escape through the outlet o and may be conducted through one or more batteries of the same kind. To the open compartment of the vessel V air has free admission.” [Reproduced with permission from ref 1. Copyright 1896, U.S. Patent Office, Washington, DC.]
10.1021/ie100902t 2010 American Chemical Society Published on Web 10/12/2010
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Figure 2. Publications and patents on “fuel cells”. A total of more than 35 523 papers (according to the Web of Science) and 130 612 patents (according to the CAPLUS database) have been published during the period of 1960-2009 (using data compiled on February 23, 2010).
Figure 3. Flow scheme of the alkaline fuel cell (AFC) system used as an onboard supply of electricity in space vehicles in the U.S. Apollo space program.
energy is ∼90% in the case of a hydrogen fuel cell, which is an extremely high value, compared to the Carnot process. It is this thermodynamic premise and the general elegancy of the fuel cell principle that have motivated generations of scientists, engineers, companies, and governments to invest vast amounts of their time and money on the technical realization and implementation of fuel cells. Figure 2 shows the rate of growth in the literature and patents over the last 50 years. One of the most prominent examples for the successful application of fuel cell technology was the use of an alkaline fuel cell (AFC) system as an onboard supply of electricity in the U.S. Apollo space program that was started in the 1960s (see Figure 3). The system was designed as three stacks, each consisting of 31 single cells operated at a temperature of 200-230 °C, using pure oxygen and hydrogen as feeds.2 The total energy output of this AFC system was 500 kWh during a 10-day mission which, at a total system mass of 810 kg, corresponds to a specific energy of 620 Wh/kg. Interestingly, 4
tons of lithium-ion batteries would be required to obtain an equivalent energy output! In the early period of this success story (1960-1975), research activities were conducted by a relatively small group of experts, as can be seen from the low number of publications in Figure 2. During this period, scientists investigated several competing fuel cell concepts that were based on different types of liquid electrolytes (see Figure 4), namely, potassium hydroxide solutions for alkaline fuel cell (AFC) systems, phosphoric acid for phosphoric acid fuel cell (PAFC) systems, and eutectic mixtures of molten carbonates for molten carbonate fuel cell (MCFC) systems. Later (since the mid-1970s), several groups started to explore the potential of solid materials as electrolytes for fuel cells, in particular, proton conducting polymers for polymer electrolyte fuel cell (PEFC) systems and ceramic oxide materials for solid oxide fuel cell (SOFC) systems. Then (from the mid-1980s onward), fuel cellssbeing a smart technology with low emissions and potentially high efficiencies, and, thus, a low consumption of primary energyshave been discussed as a very attractive traction technology for future vehicles, and as a highly efficient technology for stationary power plants. Moreover, the upcoming vision of an “hydrogen-based society” became a strong driving force for fuel cell research activities. Already, at an early stage of the fuel cell historical development, it was realized that the limited availability and the difficult handling of hydrogen might become a critical factor for fuel cell applications. Thus, following Grove’s famous words that every chemical reaction can be used to produce a voltaic current, many research groups started to investigate the direct electrochemical oxidation of alternative substances such as methanol (convertible in direct methanol fuel cell (DMFC) systems), ethanol, and dimethyl ether. However, because of the fact that, similar to that observed with hydrogen, all these substances are not readily available as pure species in today’s fuel distribution networks, many research activities were started for converting conventional fuels (natural gas, gasoline, diesel, kerosene, coal) into hydrogen-rich gas mixtures for the operation of fuel cells. More recently, biofuels such as biogas, biodiesel, wood, and silage also have received some attraction as hydrogen sources. In addition, very recently, glucose has been discussed as an attractive substance for operating medical devices via implantable enzymatic fuel cells (EFCs), with power outputs in the mWel range). As illustrated in Figure 5, apart from the more exotic field of implantable devices, fuel cells can be used as electrical energy sources for portable systems in consumer electronics and military applications (with power outputs of 1-50 Wel), as auxiliary power units, e.g., for onboard electricity generation in cars, trucks, or aircrafts (with power outputs of 1-10 kWel), for traction of vehicles, buses, and submarines (with power outputs of 160 °C. As thoroughly discussed by Zhang et al.,146 at higher temperatures, not only are the rates of electrochemical kinetics improved, but also the water management and cooling are simplified, useful waste heat can be recovered, and lower-quality reformed hydrogen can be used as the fuel. This approach has led to the development of the high-temperature polymer electrolyte fuel cell (HT-PEFC), which requires the development of new polymer electrolyte membrane materials. 3.3.3. Polymer Electrolyte Membranes (PEM). The most important properties that a polymer electrolyte should have are high thermal, thermo-oxidative, and hydrolytic stability; high proton conductivity; a low degree of swelling; and a low humidification level. The best established PEMs are based on perfluorosulfonic acid (PFSA) polymer electrolytes (having tradenames of Nafion, Flemion, Aciplex, Dow). PFSA membranes are composed of carbon-fluorine backbone chains with perfluoro side chains that contain sulfonic acid groups. The Teflon-like backbone gives these materials excellent long-term stability in oxidative atmospheres, as well as in reductive atmospheres. A lifetime of more than 60 000 h has been attained with commercial Nafion membranes at practically relevant PEFC operating conditions. The protonic conductivity of PFSA membranes is strongly dependent on the hydration state and temperature. If they are fully hydrated, the conductivity of these materials is ∼0.2 S/cm at 85 °C (Nafion 117). Consequently, for a typical membrane thickness (175 µm, Nafion 117), the electrical resistance of the PEM is 0.0875 Ω cm2, which creates a voltage loss of ∼66 mV at a current density of 750 mA/cm2. This voltage drop increases significantly if the Nafion membrane is only partially hydrated. With regard to the contact with carbon-supported noble-metal catalysts for oxygen reduction, PFSA electrolytes lead to higher catalytic activity than phosphoric acid, because of the nonadsorbing nature of the sulfonic acid anions on the Pt catalyst surface. Moreover, the solubility of hydrogen and oxygen are also found to be 20-30 times higher in PFSA electrolytes than that in phosphoric acid. As a result of the fast electrode reaction kinetics, the performance of PEFCs is high, especially at low noble-metal loadings. PFSA membranes also serve as anode/ cathode gas separators. The permeability for oxygen and hydrogen is of the order of 10-11-10-10 mol/(cm s bar), corresponding to a crossover current of 1-10 mA/cm2, which is an acceptable loss of performance. However, it is important to mention here that the crossover current for PFSA membranes is also dependent on the partial pressure difference across the membrane. Despite the high conductivity and stability, classical lowtemperature PFSA electrolyte membranes impose many restrictions on the design of efficient PEFC systems. To overcome these restrictions, alternative membrane materials must be developed for operation at temperatures above 100 °C. Important advantages of this approach include the following: (1) The kinetics for both electrode reactions is enhanced. (2) Above its boiling point, water is being transported as vapor within the PEFC; thus, the controllability of water transport is much better than that in the two-phase regime. (3) The cooling system can be simplified, because of the increased temperature difference between the fuel cell and the coolant. (4) The CO tolerance of the anode catalyst can be drastically enhanced from 10-20 ppm CO at 80 °C to 1000 ppm CO at
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130 °C, and up to 30 000 ppm CO at 200 °C. This directly leads to reduction of the complexity of the fuel processing system (i.e., in the ideal case, reformate gas from the steam reformer can be fed directly to the PEFC). (5) Higher operating temperatures enable integration of the PEFC with the fuel processing system or with hydrogen desorption from a high-capacity hydrogen storage tank. Three main classes of high-temperature proton conducting membrane materials have been developed:139-146 (i) modified PFSA membranes which incorporate hygroscopic oxides and solid inorganic proton conductors, (ii) alternative sulfonated polymers, and (iii) acid-base polymer membranes. With regard to modified PFSA membranes that incorporate hydroscopic oxides and solid inorganic proton conductors, PFSA membranes have been modified by incorporation of inorganic compounds into the hydrophilic domains (e.g., SiO2,147-149 TiO2,149 WO3,149 ZrP150,151). Thereby, nanocomposite materials with improved mechanical strength, thermal stability, and water retention properties at elevated temperatures can be obtained. For example, a H2-O2-fed PEFC equipped with a 6% SiO2-modified Nafion 115 membrane has yielded 0.4 V at 1000 mA/cm2, 130 °C, and 3 bar (total pressure).152 Considering alternatiVe sulfonated polymers, this approach is based on the sulfonation of thermally resistant poly(arylene) materials, such as sulfonated poly(ether ether ketone) (PEEK),153-155 poly(imide) (PI),156,157 polysulfones (PSF),158,159 poly(p-phenylene),160 and poly(phenylene sulfone) (PSO2).161 After sulfonation, the latter polymer showed high thermal, thermooxidative, and hydrolytic stability, as well as lower solubility and degree of swelling in water, compared to other sulfonated poly(arylene)s. This allows for the preparation of membranes with high ion exchange capacity and reasonable proton conductivity, e.g., 0.03 S/cm at 130 °C (40% RH) for sulfonated PSO2, compared to a proton conductivity of 0.15 S/cm under the same conditions for the PFSA electrolyte Nafion. However, this conductivity value is still too low to allow for acceptable fuel cell performance at elevated temperatures. With regard to acid-base polymer membranes, such as phosphoric acid-doped poly(2,5-benzimidazole) (PBI), in this approach, water is replaced with another proton-transportassisting solvent that possesses a higher boiling point (e.g. phosphoric acid and imidazoles). PBI membranes that were prepared by doping and casting from a solution of poly(2,5benzimidazole)/phosphoric acid/methanesulfonic acid (MSA) contained up to 3.0 H3PO4 molecules per PBI repeating unit. A maximum conductivity of 1.5 × 10-2 S/cm was observed at temperatures as high as 180 °C under dry conditions.162 Phosphoric acid-doped PBI was also combined with inorganic proton conductors such as zirconium phosphate,163 phosphotungstic acid,163,164 and silicotungstic acid.165 For a PBI composite that contained 15 wt % of ZrP,163 a conductivity of 9.6 × 10-2 S/cm was obtained at 200 °C and 5% relative humidity (RH). More-detailed data were summarized by Zhang et al.146 Besides optimization of the key membrane properties (conductivity, stability, swelling, water uptake), degradation of the polymer electrolyte via the formation of hydrogen peroxide, and membrane contamination by trace cationic ions (e.g., Fe2+, Cu2+) are issues of very high practical importance.166 Many cationic species exhibit a high affinity for the sulfonic groups in Nafion membranes. This can lead to a lower proton conductivity and, therefore, an increased membrane polarization. Cation exchange with protons inside the membrane can also result in membrane dehydration and water management prob-
lems. In addition, the presence of minor impurities of Fe2+ and Cu2+ can accelerate the decomposition of the electrolyte membrane, because of the formation of oxygen radicals that are caused by the reaction with hydrogen peroxide. 3.3.4. Membrane Electrode Assemblies (MEA). The MEA is a sandwich structure that consists of the polymer electrolyte membrane, coated with anode and cathode catalyst layers, combined with diffusion layers, which act as current collectors and also as porous structures for mass transport of reactants and products. Understanding and controlling the charge, mass and energy transport mechanisms within the MEA is a key issue for the design of highly efficient PEFCs. In this field, mathematical models can be very helpful for guiding the development of novel materials and structures. Pioneering influential modeling works were performed by Springer et al.167 and Bernardi et al.,168,169 who have studied the factors that limit the cell performance and analyzed the mechanisms of species transport in the complex phase network of gas, liquid, and solid phases within the MEA of the PEFC. Today, mathematical modeling is a standard tool that is used (1) to study the interaction of transport phenomena and electrochemical reactions in catalyst layers and to optimize the structure and composition of these layers;170-177 (2) to understand the two-phase flow and multicomponent transport in the PEFC cathode;178-186 (3) to study the transport of charged and noncharged species in PEFC membranes (conduction, diffusion, electro-osmotic drag, crossover transport);187-195 and (4) to understand the complex interactions of mass transport, heat transport, and electrochemical reactions across the entire MEA with all its layers.196-200 With regard to MEA preparation, in the past, the production was dominated by recipe-driven catalyst-ionomer ink formulations and manufacturing procedures developed empirically as individual solutions in certain laboratories and companies. Major advances were made in the fabrication starting from the PTFEbound catalyst layers of almost 20 ago to the present thin-filmon-membrane catalyst layers, which consist of two interpenetrating networks of electron and proton conductors. The benefit of the latter approach is the expansion of the interfacial region between catalyst particles and ionomer.201 The content of the ionomer in the catalyst layer is a key parameter for optimizing the MEA structure and performance.202 Thereby, a significant reduction of the platinum content, to
